Photoelectric sensors use light to sense targets without physical contact and are used in a wide variety of applications and environments, such as to sequentially detect the presence or absence of targets on a conveyor belt, or to detect a change in the size, shape, reflectivity, or color of a target. Various types of photoelectric sensors are available, such as transmitted beam sensors, retro-reflective sensors, and diffuse sensors. Each of these sensors includes a light source, such as a light emitting diode (LED) or a laser, and a photodetector for detecting light, such as a photodiode or phototransistor, and can also include one or more lenses to focus or narrow the beam of light emitted by the light source and/or to focus or narrow the received light for efficient detection by the photodetector. These sensors typically also include circuitry in communication with the photodetector for producing a voltage or current signal indicative of a characteristic of the sensed target, such as high and low voltage or current states for respectively indicating the presence and the absence of the target at a specified location.
A transmitted beam photoelectric sensor is arranged such that the light source is located on one side of a path of a target to be sensed, and the photodetector is located on the other side of the path. A light beam from the light source is directed to the photodetector, and when the target blocks this light beam from being received by the photodetector, a resulting change in the amount of detected light gives rise to an output signal indicative of the presence of the target. As for the retro-reflective and diffuse type sensors, both of these sensor types combine the light source and the photodetector in a single housing. A retro-reflective sensor uses a reflector situated on the opposite side of a path of a target to be sensed, and the reflector reflects a light beam from the light source back to the photodetector, with the presence of the target blocking this light beam. A diffuse sensor operates by using the target itself to reflect the light beam from the light source back to the photodetector such that, for example, more light is received and detected when the target is present compared to when it is absent. Diffuse sensors are well suited for applications with space requirements that limit the positioning of a reflector across from the photodetector.
Successful sensing requires that a change in the position, size, shape, color, or reflectivity of the target causes a sufficient measurable change in the amount of light detected by the photodetector. The performance of a photoelectric sensor can be quantified using the concept of margin or excess gain. Margin is a measurement of the amount of light from the light source that is detected by the photodetector compared to a minimum light level required to switch the output signal of the sensor (such as from a level indicative of the absence of a target to a level indicative of the presence of a target). A margin of zero occurs when none of the light emitted by the light source can be detected by the photodetector. A margin of one occurs when just enough light is detected to cause the output signal of the sensor to change states, for example, to switch from a low level indicative of the absence of a target to a high level indicative of the presence of the target. A margin of twenty (commonly expressed as 20×) occurs when twenty times the minimum light level required to switch the output signal of the sensor is detected by the photodetector. In other words, the higher the margin, the more capable a photoelectric sensor is at sensing a target at that distance.
Margin is measured and expressed relative to the reflectivity of the reflecting surface, for example relative to a white paper having a reflecting surface rated at 90% reflective, which will reflect more light and therefore allow for a larger margin than a paper surface that is 18% reflective. A margin value corresponds to a specific distance between a target to be sensed and the front of the receiving lens of the sensor, and typical response curves are often provided for a photoelectric sensor, which show what the typical margin will be depending on the sensing distance. A sensing distance for a diffuse type sensor is defined as the distance from the front of the receiving lens of a sensor to the specified target.
Photoelectric sensors are often characterized in terms of their maximum and minimum sensing distances. Often a “blind area” exists in which a target that is too close to the sensor can not be sensed because the light reflected from the target is not received by the photodetector. This occurs in the case of a diffuse sensor because the light source and the photodetector are combined within the same housing such that the emitted light and the light reflected from the target to be detected and received by the sensor each travel along different paths, and the reflected light enters the sensor at an angle with respect to the emitted light. The closer a target is to the sensor, the greater is the angle of the reflected light. At some point, the angle between the paths becomes so great that no light can be received by the photodetector.
In the case of target detection, in order to obtain a sufficient measurable change in the amount of received light by the photodetector from the target reflecting light emitted by the light source (or blocking it), it may be necessary to increase the intensity of the emitted light, to increase the sensitivity of the photodetector, and/or to increase the diameter of a receiver lens situated in front of the photodetector. These modifications allow more light to be detected by the photodetector and result in an increased margin.
The drawback to these modifications is that background (non-target) targets are more likely to reflect light back to the photodetector, resulting in inaccurate measurements corresponding to the target to be sensed. In addition, increasing the size of the receiver lens will effectively increase the target minimum sensing distance, thereby limiting the photoelectric sensor's ability to sense targets situated close to the photoelectric sensor. Specifically, for targets that are situated very close to a diffuse type photoelectric sensor, the reflected light received is at such a wide angle with respect to the emitted light that it is not received or sensed by the photodetector. Further, the size of the receive lens is limited by the size of the photoelectric sensor itself.